STMicroelectronics Makes 3-Axis Digital Gyroscope With One Sensor

Yaw, pitch, and roll, all from one MEMS element

25 March 2010—Nowadays, a phone that doesn’t know where it is or where it’s going can’t really call itself ”smart.” To orient themselves properly, smartphones require not just GPS capability but also an electronic compass, an accelerometer, and increasingly, digital gyroscopes.

The point of a gyroscope is to sense any change in an object’s axis of rotation. Up until now, gyroscopes measured movement around the three axes with three sensors—one for pitch, one for yaw, and another for roll. At most, two of these sensors would be combined on a single die. The best you could do was, say, match up a 3- by 5- by 1-millimeter yaw sensor with a 4- by 5- by 1-mm sensor that would detect pitch and roll. But on 15 February, STMicroelectronics unveiled a 4- by 4- by 1-mm gyroscope whose single sensing structure tracks all three angular motions. It’s a triumph of microelectromechanical systems (MEMS) engineering.

”Cellphone companies continually demand smaller size, less power, and lower cost,” says Jay Esfandyari, MEMS product marketing manager at ST. ”The aim now is to eventually shrink the [gyroscopes] significantly in the near future, down to about the [3-mm square] average footprint of accelerometers” typically used in smartphones.

The new 3-axis digital gyroscope comes preset with one of three sensitivity levels, which allow the device to trade speed for resolution. For gaming, it can capture movements as quick as 2000 degrees per second but can distinguish only movements larger than 70 millidegrees per second per digit. For the more controlled point-and-click movement of a user interface—say a wand or a wearable mouse—the gyroscope can pick up movements as fast as 500 degrees per second and distinguish movements of 18 millidegrees per second per digit or more. The most sensitive version, which picks up only 250 degrees per second, can sense a mere 9 millidegrees per second per digit.

The benefits of lightning-fast sampling rates are notable in video games in which avatars’ movements are controlled by a wand, such as those for Nintendo’s Wii gaming system. A wand featuring only a 3-axis accelerometer can measure static tilt angles for pitch and roll, but these measurements will have big errors under dynamic motions, says Esfandyari. Worse yet, he says, these errors tend to accumulate over time, throwing off the system’s accounting of angular displacement. The result is zigging when the player wants to zag and pounding when the player intends a light tap. Combining a 3-axis gyroscope with a 3-axis accelerometer gives game play a much smoother feel while making errors virtually unnoticeable.

ST is planning to integrate a gyroscope and an accelerometer into a single inertial measurement unit, says Esfandyari. This, he says, will make motion tracking even more accurate, because data regarding acceleration and angular displacement rate will be reported at the exact same time.

The gyroscope also represents a high-water mark in terms of energy consumption, according to ST. The new 3-axis digital device draws only 6 milliamps; two years ago, ST’s single-axis gyroscopes drew 5 mA. The new device gets more done with less energy because it operates in ”sleep mode.” The driving structure of the device is always on, but the sensing structure is off when a gadget’s direction-finding function is not in use. When the gyroscope is needed, the sensing structure can come back on and be ready to record movement readings in less than 20 milliseconds.

ST says it’s in a position to further reduce that delay in the near future should the market require it. Esfandyari says that minimizing the time required for the sensing structure to turn on and off is critical in applications such as dead reckoning, in which the device plots its progress on a map purely by the way it senses motion. This means that missing the initial movements will almost guarantee that every subsequent reading will be in error.